(1) Field of the Invention
The present invention relates to vertical semiconductor chips or devices including GaN, GaInP, and GaInNP based LEDs, especially for high power LEDs, and methods of mass production of the same.
(2) Prior Art
There are varieties of prior art discussing vertical semiconductor devices or chips and method of manufacturing the same. Main advantages associated with the vertical structure comprise the following: (1) vertical semiconductor devices or chips have all of advantages of flip chip technique including fast heat dissipation and without its disadvantages, such as complex process steps, low throughput, and expensive equipments; (2) two electrodes are on the two opposite sides of a vertical semiconductor device or chip and, thus, current distributes more uniformly, current density is higher, ESD property is improved.
U.S. Pat. No. 6,320,206 by Coman, et al. disclosed methods for removing sapphire substrate for GaN based LEDs by laser lift-off. However the electrical and optical properties of LEDs are depended on crystal quality of the epitaxial layers that is subjected to additional chemical process steps (e.g. etching), mechanical process steps (e.g. lapping), and laser lift-off process steps. It may be needed for laser beam to scan an epitaxial wafer more than one times, which decreases the throughput and increases the chance of damaging the epitaxial layer of LEDs. Also the laser lift-off equipment is expensive.
For removing original substrates and growing a thick semiconductor layer as a semiconductor substrate for further application of semiconductor devices including LEDs, U.S. Pat. No. 6,679,947 by Koike et al. disclosed a method comprising following process steps: disposing a metal layer having low melting point on the original substrate, growing a semiconductor substrate of about 100.mu.m thick on it, and then removing the original substrate by dissolving the metal layer and separating the semiconductor substrate from the original substrate. To grow a thick semiconductor substrate in this way, however, is very expensive. Also semiconductor devices or chips grown on the so-obtained semiconductor substrate do not have the advantages of flip chip technique.
It is important to maintain a uniform temperature over the surface of a substrate being processed in epitaxial growing. U.S. Pat. No. 6,492,625 disclosed the apparatus and method for controlling temperature uniformity over the whole surface of the substrate. Substrate temperature non-uniformity may result in deviation from target wavelength and intensity of emitted light. In a worse case scenario, the edge of a sapphire substrate may slightly warp and bowl and, thus, not contact to the susceptor well during the epitaxial growth in MOCVD. As a consequences, the composition and quality of disposed epitaxial layer over the substrate is not uniform. Therefore, LED chips on an about 5 mm ring from the edge of the substrate are not qualified, i.e., the yield is lower.
When a GaN epitaxial layer growing on an original substrate at about 1050 degree C., there is no stress due to the different in the thermal expansion coefficients of the original substrate and the epitaxial layer. Then the temperature is cooled down to room temperature of about 20 degree C., both the epitaxial layer and the original substrate constrict for a temperature range of about 1000 degree C. Therefore the different in the thermal expansion coefficients of the original substrate and the epitaxial layer causes a stress which affects the crystal quality of the epitaxial layer.
Therefore there are increasing demands for methods of cost effectively and massively producing vertical semiconductor chips or devices with higher crystal quality and without disadvantages mentioned above.